Electrophotographic printing is a non-impact printing technology invented by Chester Carlson in the 1930s. It occupies a large segment of the total printing market, with a global market value of $59.9 billion in 2009. Electrophotographic printing is a highly complex printing technology consisting of 2 core components, namely the photoconductor (PC) and the toner. The printing process involves 7 distinct steps, which include PC charging, PC exposure, toner development, toner transfer, fusing, cleaning, and charge erasure. The photoconductor, as a primary component, is involved in 6 of the 7 aforementioned steps. Thus, both photoconductor durability and performance are highly sought-after characteristics.
An example of a process applied for forming images by electrophotography using these photoconductors is the Carlson process, named after Chester Carlson. In this process, image formation is carried out by charging the photoconductor by corona discharge in the dark, forming an electrostatic latent image such as characters or pictures of a copy on the surface of the charged photoconductor, developing the formed electrostatic latent image with toner, and fixing the developed toner image on a carrier such as paper, and following transfer of the toner image, the photoconductor is reused after carrying out erase, removal of residual toner and optical erase.
The photoconductor is the component through which a latent image can be formed, with the latent image being developed by toner particles in the subsequent step. Initially, an electrostatic charge is distributed through projection on the surface of the PC. Next, light exposure results in generation of charge carriers within the PC and through absorption of light by the CGM. The charge carriers are transported to the PC surface and the opposite electrode by CTM. As the charge carriers reach the surface, they neutralize surface charges within the area previously illuminated. This forms a latent image on the surface of the PC, which can then be subjected to toner development.
Photoconductors are required to retain surface charge in the dark, and must be able to transport a charge by absorbing light. Single-layer photoconductors possess both of these functions in a single layer of the photoconductuor. Multilayer photoconductors separate these functions into separate layers: a charge generation layer and a layer that retains the surface charge in the dark and transports the charge during absorption of light.
Photoconductor performance relies on several factors, including charge acceptance during projection of charge on PC surface, free charge generation and transport following illumination, and the degree of surface charge neutralization. All these factors work in concert to exemplify the overall performance of a photoconductor. The performance is typically measured in terms of sensitivity of the photoconductor to light exposure at a particular wavelength, with higher photosensitivities associated with enhanced PC performance. Additionally, the performance can be measured in terms of the rate of photodischarge of the photoconductor once illuminated with light of specific wavelength, with higher discharge rates associated with a better photoconductor.
Of special importance is the charge generation material (CGM) incorporated in a photoconductor. Desired CGM characteristics include efficient absorption of light at the exposure wavelength, low recombination of initially-generated charges, the ability to produce free charges and transfer charges to transport material, and photostability. As such, both the optical/electronic properties of the CGM and manipulation of these properties through the choice of correct material and environment are of utmost importance. In addition, PCs are required to be manufactured in a cost-effective manner, so to reduce the overall cost of the printing device.
Current photoconductors utilize dyes such as diazo or phthalocyanine compounds and derivatives as CGM. These compounds are readily available and have been produced and used as CGMs in electrophotographic printer's photoconductors extensively. Nevertheless, research and development of novel CGMs have been ongoing, due to the need for charge generation material with increased photoresponse (resulting in higher printing speed), and higher photostability (resulting in longer lifetime).